Oxygen is separated from oxygen containing feeds, such as air, through cryogenic rectification. In order to operate a cryogenic rectification plant, refrigeration must be supplied to offset ambient heat leakage, warm end heat exchange losses and to allow the extraction or production of liquid products, including liquid oxygen, liquid nitrogen, or liquid argon. While the main source of refrigeration for a cryogenic rectification plant is typically supplied by expanding part of the feed air stream or a waste stream to generate a cold stream that is then introduced into the main heat exchanger or the distillation column, external refrigeration can also be imparted by other refrigerant streams introduced into the main heat exchanger, including a refrigerant stream from a closed loop supplemental refrigeration cycles as described in U.S. Pat. No. 8,397,535.
One of the limitations or drawbacks of the existing closed loop refrigeration cycles used in air separation plants is that the centrifugal compressors and turbo-expanders in such supplemental refrigeration circuits involve additional capital costs that when operating, are generally operating in an ‘on’ or ‘off’ mode. In other words, the centrifugal compressors and turbo-expanders are either operating so as produce the supplemental refrigeration and additional liquid product make or are shut down thereby not producing supplemental refrigeration or foregoing any additional liquid product make. The continued cycling of the centrifugal compressors and turbo-expanders between operating mode and shut-down mode adversely impacts the overall efficiency and reliability of the supplemental refrigeration cycle.
A small degree of adjustment in existing supplemental refrigeration circuits may be achieved through the adjustment of compressor inlet guide vanes. However, one must be careful of adjustments that would sent the compressor into a surge condition or a stonewall conditions as a result of too little or too much flow to the compressor. As a result, the existing or prior art supplemental refrigeration circuits are generally operated at a fixed or near-fixed operating point. This inability to modulate the level of supplemental refrigeration over broad operating ranges effectively limits the plant operator from precisely controlling the amount of liquid product produced by the air separation plant at any given time.
Another challenge to the use of closed loop refrigeration circuits is encountered when integrating such closed loop refrigeration circuits into the design of a cryogenic air separation plant and associated air separation cycle. Such integration typically requires changes to one or more of the main air compression train, the main heat exchanger, the distillation columns, and/or the turbine expansion based refrigeration circuits of the air separation plant. In addition, for some cryogenic air separation plants there is a need to design the refrigeration and liquefaction process that avoids or defers some of the up-front capital costs associated with the closed loop refrigeration cycles but allows such supplemental refrigeration to be easily added to the cryogenic air separation plant at a later date after construction of the air separation plant when the liquid product requirements change.
What is needed, therefore, is a closed loop refrigeration circuit that can be easily retrofitted to an air separation plant at a later date to address the upfront capital cost and design challenges associated with closed loop refrigeration circuits. Once installed, the add-on closed loop refrigeration circuit should be capable of modulating the level of supplemental refrigeration produced over broad operating ranges and thus allows more precise control of the amount of liquid product produced by the air separation plant.